Fix parent of root directory turning into ./ on Windows (#7803)

libretro-common/file/file_path.c

@@ -693,6 +693,7 @@ bool fill_pathname_parent_dir_name(char *out_dir,
  *
  * Copies parent directory of @in_dir into @out_dir.
  * Assumes @in_dir is a directory. Keeps trailing '/'.
+ * If the path was already at the root directory, @out_dir will be an empty string.
  **/
 void fill_pathname_parent_dir(char *out_dir,
       const char *in_dir, size_t size)
@@ -779,12 +780,26 @@ void path_basedir(char *path)
  *
  * Extracts parent directory by mutating path.
  * Assumes that path is a directory. Keeps trailing '/'.
+ * If the path was already at the root directory, returns empty string
  **/
 void path_parent_dir(char *path)
 {
+   bool path_was_absolute = path_is_absolute(path);
    size_t len = strlen(path);
    if (len && path_char_is_slash(path[len - 1]))
+   {
       path[len - 1] = '\0';
+      if (path_was_absolute && find_last_slash(path) == NULL)
+      {
+         /* We removed the only slash from what used to be an absolute path.
+          * On Linux, this goes from "/" to an empty string and everything works fine,
+          * but on Windows, we went from C:\ to C:, which is not a valid path and that later
+          * gets errornously treated as a relative one by path_basedir and returns "./".
+          * What we really wanted is an empty string. */
+         path[0] = '\0';
+         return;
+      }
+   }
    path_basedir(path);
 }
 

libretro-common/include/file/file_path.h

@@ -136,6 +136,7 @@ void path_basedir(char *path);
  *
  * Extracts parent directory by mutating path.
  * Assumes that path is a directory. Keeps trailing '/'.
+ * If the path was already at the root directory, returns empty string
  **/
 void path_parent_dir(char *path);
 
@@ -314,6 +315,7 @@ bool fill_pathname_parent_dir_name(char *out_dir,
  *
  * Copies parent directory of @in_dir into @out_dir.
  * Assumes @in_dir is a directory. Keeps trailing '/'.
+ * If the path was already at the root directory, @out_dir will be an empty string.
  **/
 void fill_pathname_parent_dir(char *out_dir,
       const char *in_dir, size_t size);